Electron emissive cathode



May 20 1941,. w. HARTMANN mL. 2,242,395

LEGTRON EMIssIvE cATHoDE- Filed June 14 19'59 IIIIIIIIIIIIIIIIIIIIII iiilIllIIIIIIIIIIIIIIIIIIIIIIII 7/////////////////////// W. HA/PrMA/VN NVENTO' H. WULFHEKEL Jagt //gwk ATTORNEY Patented May 20,1941

nLEcraoN Emssrvicarnonn werner Hartmann, Berlin-Zehlendorf, and Herbert Wulfhekel, Berlin-Steglitz, Germany, assigners to Fernseh Aktiengesellschaft, Berlin- Zehlendorf, Germany, a company of Germany Applicatilcgi June 14, 1939, Serial No. 279,028

Germany June 18, 1938 9 claims. (ci. 25a-27.5)

This invention relates to electron emissive cathodes, particularly to photocathodes such as are used in the photoelectric tubes, picture analyzers with a continuous photoelectriclayer or a mosaic, especially for television purposes, and

to secondary emitting layers such as are 'usedA in electron multipliers.

It is known to form surfaces preferably of silver and caesium, and to evaporate upon the finished surface a metal such as antimony or bismuth which lfavorably iniiuences the photosensitivity. A'greater sensitivity isv then obtained, particularly in the blue-green range. In operation of such cells it has been found that their spectral sensitivity becomes displaced after a certain time and changes in absolute values. This phenomenon is traced back to the fact that the antimony atoms diffuse from the surface into the interior of the layer. It has been observed that the diffusionvelocity of antimony atoms at 200 degrees C. is unexpectedly great. Even if the diffusion velocity decreases exponentially with temperature, it still is of considerable magnitude even for the operating temperature of the cell, which is at least room temperature. Inasmuch as the emission of photoelectrons is a surface eiect penetrating only slightly below the surface, a displacement in the order of several atoms is suiilcient to considerably change the properties of the layer.

According to the invention, this displacement is avoided by decreasing themagnitude of the concentration gradient of the subsequently evaporated metal, for instance, antimony. Hereby it is accomplished that less antimony atoms diffuse at lower speed from the surface of the photosensitive layer into the layer.

In some cases it sumees to provide a blocking layer of a material with a nner crystalline structure, such as copper, to prevent the diusion of antimony atoms. .l The invention will now be described more in detail having reference to the accompanying drawing:

Fig. 1 illustrates one form of photoelectric cathode according to this invention;

Fig. 2 illustrates a modified form of cathode,

The invention may be explained with the aid of two embodiments.' In Fig. 1, numeral l indicates the cathode carrier, for instance a por- `tion of the glass wall of the photoelectric tube. An antimony layer 2 is evaporated upon this glass wall. A comparatively thin layer 3 of silver is then evaporated upon the antimony layer 2. The silver is then oxidized in the known manner and treated with caesium and antimony. atoms will diffuse from the foundation into the silver layer and thus decrease the concentration gradient for the antimony atoms evaporated upon the silver-silver-oxide-caesium layer.

Fig. 2 shows .the series of layers for another embodiment of the invention. In this case a.A silver layer 4 is deposited upon the cathode carrier l, and the silver layer is oxidized in the usual manner. A thin layer S of antimony is then evaporated upon the silver-oxide layer B.

The caesium atoms of a layer of caesium deposited upon the layer t of antimony will more or less mix with the antimony atoms so that antimony atoms are present on the surface of the' photosensitive layer forming layer i. In this supporting surface, a layer of bismuth on said` case a reduction inl concentration gradient is also accomplished. -It may even be possible to completely compensate the concentration gradient.

Photocathodes, produced by the method according to the invention possess at least the same sensitivity and spectral distribution as known photocath'odes sensitized with antimony and possess ,a considerably longer lifetime. Of course, this method is not limited tocathodes which are sensitized with antimony, but is` applicable in the same manner also to such cathodes in which sensitization takes place, for instance, with bis- I muth.

What is claimed is: 1. An electron emissive cathode comprising a supporting surface, a layerof antlmony on said surface, a thin layer of silver on said layer of antimony, and a matrix on said silver layer comprising caesium, caesium oxide, and antimony.

2. An electron emissive cathode comprising a surface, a thin layer of silver on said layer of bismuth; and a matrix on said silver layer comprising caesium, caesium oxide. and bism th.

3. An electron emissive cathode comp ing a supporting surface, a layer of a metal of the group containing antimony and bismuth, a thin layer of silver on said first layer, anda surfacematrix on. said silver layer comprising an alkali metal and a metal of said group containing 4. A method of forming an electron emissive cathode which comprises evaporating and condensing antimony to form a layeron a cathode carrier, evaporating and condensing a small amount of silver to form a thin layer of silver on said layer of antimony, and forming on said During the forming process antimony:

cathode which comprises forming a layer of a metal of the group containing antimony and bismuth, forming on said layer a thin layer of a metal of the iirst group of the periodic table oi.' the elements exclusive oi' the alkali metals, and forming on said second layera matrix comprising an intimate mixture of an alka1i-metal and a metai of said group containing antimony and bismuth.

'1. A method of forming an electron emissive cathode which comprises evaporating and condensing bismuth to form a layer on a cathode carrier, evaporating and condensing a small ,amount of silver to form a4 thin layer of silver on said layer of bismuth, and forming on said silver layer a matrix comprising an intimate/f' mixture oi' alkali metal and bismuth.

,8. A method or forming an electron emissive 4cathode which comprises evaporating and condensing bismuth to form a layer on4 a cathode carrier, evaporating and condensing a small amount oi* silver to form a thin layer oi silver on said layer of bismuth, oxidizing the surface oi' said silver layer. and forming on said-oxidized silver a matrix comprising an intimate mixture of caesium and bismuth.

9. An electron emissive cathode comprising a layer of an electronemissive metal sensitized with a metal of the group containing antimony and bismuthV and an underlying layer of a metal of the group containing antimony and bismuth to minimize the concentration gradient of said sensitizing metal.

HERBERT WUIFHEKEL. WERNER HARTMANN. 

